Laser interferometer having a sheath for the laser beam

ABSTRACT

An interferometer used to measure distance to an object is provided with a laser sheath. The sheath encloses a substantial part of the measurement beam&#39;s path to provide a controlled environment which reduces environmental influences on the measured distance. The sheath is of variable length and responsive to a follower so as to maintain a sheath end nearest the object at a small distance from the object. An environmental controller controls the environment within the sheath. The environment within the sheath can be a vacuum or a suitable gas or gas mixture. A corrector can be used to compensate the interferometer&#39;s measured-distance signal for detected environmental characteristics to produce a corrected signal which indicates distance between the interferometer and the reflective surface. The apparatus and methods can be used to measure and control stage position in a projection-type wafer exposure system which is affected by variations in its atmospheric environment.

This application is a continuation of application Ser. No. 08/349,733,filed Dec. 2, 1994, now U.S. Pat. No. 5,552,888, issued Sep. 3, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to laser interferometer methods and apparatus, andmore particularly to laser interferometers useful in positioning wafersin a semiconductor production system with improved isolation of theinterferometer laser beam from environmental influences.

2. The Prior Art

FIG. 1 shows in simplified, schematic, perspective view a typicalprior-art arrangement for positioning a semiconductor wafer 100. Wafer100 is mounted in a chuck on an XY stage 105 having a bed 110 movable byoperation of a controllable positioning device 115 along the X directionand a bed 120 movable by operation of a controllable positioning device125 along the Y direction. The positioning devices and beds are mountedon a rigid platform 130. In the arrangement shown, stage 105 is used toposition wafer 100 relative to an optical image produced by a lightsource 135, a reticle 140 and a projection lens assembly 145 in astep-and-repeat wafer exposure apparatus. The position of wafer 100 ineach of the X and Y directions is measured by a laser interferometersystem having a laser source 150, a beam splitter 155, interferometers160 and 165, and mirrors 170 and 175. A laser beam 180 split by beamsplitter 155 is supplied to interferometers 160 and 165. Interferometer160 in turn splits the beam into a reference laser beam (not shown) anda measurement laser beam 185 which is applied to mirror 170 formeasuring position of wafer 100 in the X direction. Interferometer 165splits the beam into a reference laser beam (not shown) and ameasurement laser beam 190 which is applied to minor 175 for measuringposition of wafer 100 in the Y direction. The measured position is fedback to a stage control circuit (not shown) which controls positioningdevices 115 and 125 to position the wafer in a closed loop controlsystem.

Known systems of this general type have the stage, wafer and opticalsystem surrounded by atmospheric air, though typically in anair-conditioned environment to provide control of dust and airtemperature. Even so, air temperature varies enough over the length ofthe measurement laser beams, over short periods of time, to introduce asignificant error in the wafer-position measurement. These variationscan result, for example, from heat-producing components such aspositioners 115 and 125, light source 135, and/or laser source 150.

FIG. 2 illustrates the effect of environmental variation in positionmeasurement. Distance L between interferometer 160 and mirror 170 is tobe measured using a measurement laser beam 185 aligned in the Xdirection. Rather than directly measuring distance, interferometer 160measures an optical path length OPL which is related to distance L andto the index of refraction η of the air through which beam 185 passes bythe relationship

    OPL=ηL

Any variation in the index of refraction directly affects OPL and, withit, the apparent position of wafer 100. In situations where adifferential interferometer is used, some compensation is possible ifthe reference beam can be located in an environment that is comparableto the measurement beam. This can compensate large-scale environmentalchanges but does not adequately compensate localized variations. Forexample, it has been calculated that to measure the apparent distance Lof a 420 mm path length to within 1 nm, temperature and pressure of theair through which the measurement laser beam passes must be maintainedto within 0.002° K and 0.006 mm Hg, respectively. Such tolerances arenot believed achievable with existing technology.

U.S. Pat. No. 4,814,625 describes a semiconductor wafer-exposure systemsimilar to that of FIG. 1 in which air-conditioning devices are providedfor blowing currents of air at a controlled temperature toward the stagealong the measuring paths of the laser interferometer system in the Xand Y directions. Such a system attempts to limit air temperaturevariations which can affect measurement accuracy. U.S. Pat. No. 5,141,318 describes a laser-interferometer measuring apparatus and methodfor positioning a wafer in a semiconductor production system.Temperature-controlled air is passed through a vent which blows auniform, laminar flow of air over the length of the measurement laserbeam.

In practice, the systems of these two patents have at least two majordrawbacks. First, they are difficult to implement due to limited spacein the vicinity of the stage in a real wafer-stepper apparatus. Second,the improvement in measurement accuracy is believed inadequate for theexpected demands of next-generation semiconductor processes. While sucharrangements can reduce to some degree the influence of air temperatureand pressure variations on measurement accuracy, more effectiveisolation from environmental fluctuations is needed.

Electron-beam lithography systems are known in which a wafer ispositioned using a stage, and in which the electron optics and the stageand wafer are all enclosed in a vacuum chamber. While a vacuumenvironment is required for operation of an electron beam system, it isimpractical and even undesirable for a semiconductor production system.Construction and maintenance of a vacuum system large enough for awafer-stepper system would be costly and complex. Vacuum pumps, seals,housing elements and the like would increase initial cost and take upcostly production space. System maintenance would be hindered by theneed to assure vacuum sealing during system operation and after eachintervention. Air locks would be needed to avoid losing vacuum each timea new wafer is introduced, possibly limiting achievable productionrates. The changed index of refraction would mandate a complete redesignof the complex optical exposure system.

Despite its other disadvantages, the use of a vacuum environment offersthe advantage of improved interferometer measurement accuracy due toreduced environmental influences on the measurement laser beams.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the invention, aninterferometer used to measure distance to an object is provided with alaser sheath. The sheath encloses a substantial part of the measurementbeam's path to provide a controlled environment (e.g., of vacuum or of agas such as Helium) which reduces environmental influences on themeasured distance. The sheath is of variable length and responsive to afollower for maintaining a sheath end nearest the object at a smalldistance from the object. An environmental controller controls theenvironment within the sheath. A corrector compensates theinterferometer's measured-distance signal for detected environmentalcharacteristics along the beam path to produce a corrected signal whichindicates distance between the interferometer and the reflective surface(e.g., of a stage mirror). The apparatus and methods can be used tomeasure and control stage position in a step-and-repeat wafer exposuresystem, and in other applications requiring precise distancemeasurement.

In one embodiment, the sheath comprises a housing enclosing a portion ofthe beam path, the housing having a wall and a beam opening through thewall, and the sheath portion comprises an elongate hollow memberextending through the beam opening and supported for movement relativeto the wall in a direction generally parallel to the beam path. Thesheath portion is supported in the beam opening by an air-guidedbeating. An end of the sheath portion nearest the reflective surface iscovered with a window which allows passage of the measurement beambetween the controlled environment and the reflective surface. Thefollower varies the effective length of the sheath portion by displacingthe sheath portion along the beam path to maintain the window at asubstantially constant distance from the reflective surface. A gapsensor measures distance between the window and the reflective surface,a temperature sensor measures atmospheric temperature in the region ofthe gap, and a pressure sensor detects pressure within the controlledenvironment. The corrector is responsive to the gap sensor, thetemperature sensor and the pressure sensor for compensating themeasured-distance signal in dependence on air-gap length, air-gaptemperature, pressure within the controlled environment and the index ofrefraction of the window.

In another embodiment an end of the elongate hollow member nearest thereflective surface is fitted with a bearing for maintaining the sheathportion at a substantially fixed spacing from the reflective surface. Inthis embodiment, no window is required. The bearing comprises a tip endhaving a face resiliently biased toward the reflective surface andhaving at least one orifice for expelling pressurized gas to maintainseparation between the face and the reflective surface. In the case of avacuum environment within the sheath, the vacuum pumping capacity of thesystem is sufficient to exhaust the gases entering the sheath.

In a further embodiment, the sheath portion comprises a telescopingassembly of elongate hollow members, the assembly being supported fortelescoping movement in a direction generally parallel to the beam path.In yet another embodiment, the sheath portion comprises an elongatehollow member having a bellows of variable length, supported forextension and retraction in a direction generally parallel to the beampath.

These and other features of the present invention are described belowwith reference to the drawing figures, in which like components areindicated by like reference numerals.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows in schematic, perspective view a typical prior-artarrangement for positioning a semiconductor wafer 100;

FIG. 2 illustrates the effect of environmental variation in positionmeasurement in a prior art system;

FIG. 3 schematically shows a modification of the arrangement of FIG. 2in accordance with the invention;

FIG. 4 schematically shows a further modification of the arrangement ofFIG. 2 in accordance with the invention;

FIG. 5 is a partially cut-away elevation view schematically showing anarrangement for measuring and controlling position of a stage inaccordance with the invention;

FIGS. 6A and 6B illustrate the relationship of optical path length tomeasured distance in preferred embodiments of the invention;

FIG. 7 is a partially cut-away elevation view of a modified version ofthe arrangement of FIG. 5 as installed in a photolithography system inaccordance with the invention;

FIG. 8 is a perspective view of a wafer exposure apparatus having a pairof interferometer systems in accordance with the invention;

FIG. 9A is a partial, exploded, perspective view of parts of a guidelessstage,

FIG. 9B is an assembled, perspective view of a guideless stage;

FIGS. 10A-10D are a sequence of perspective views showing operatingpositions of the stage of FIG. 9B;

FIG. 11 is a perspective view of a guideless stage having interferometersystems in accordance with the invention;

FIG. 12 is an enlarged-scale view showing details of a portion of FIG.11;

FIG. 13 is a partially cut-away elevation view of a further guidelessstage having modified interferometer systems in accordance with theinvention;

FIG. 14 is an enlarged-scale view showing details of a portion of FIG.13;

FIG. 15 is a sectional view taken along line XV--XV of FIG. 14;

FIG. 16 is a sectional view taken along line XVI--XVI of FIG. 14;

FIG. 17 is an enlarged-scale, sectional view taken through lineXVII--XVII of FIG. 18 and showing internal structure of an air-guidedbearing in accordance with the invention;

FIG. 18 is an end view of the air-guided bearing in accordance with theinvention, with hidden lines to show internal structure;

FIG. 19 is a partially cut-away elevation view of a further arrangementfor measuring controlling stage position in accordance with theinvention;

FIG. 20 is a sectional view of a further vacuum sheath embodiment inaccordance with the invention;

FIG. 21 is a partially cut-away elevation view of an embodiment of theinvention comparable to that of FIG. 5, showing horizontal extent of thesystem along an axis;

FIG. 22 is a partially cut-away elevation view of an embodiment of theinvention modified to reduce the system footprint and showing the lasersheath retracted;

FIG. 23 is a partially cut-away elevation view of the embodiment of FIG.22 with the laser sheath in an extended position; and

FIG. 24 is a partially cut-away elevation view of a guideless stagehaving a gas-filled laser sheath in accordance with the invention.

DETAILED DESCRIPTION

FIG. 3 shows an idealized modification of the arrangement of FIG. 2 foravoiding the influence of environmental variations on the apparentposition of wafer 100. A sheath 300 surrounds measurement laser beam 185and is evacuated by a vacuum pump 305. To measure the apparent distanceL of a 420 mm path to within 1 nm in a vacuum, for example, thetemperature and pressure of the air through which the measurement laserbeam passes must be sensed to a tolerance of±1.54° K and ±0.00618 mm Hg,respectively. Such tolerances are readily achievable in a vacuumenvironment of ≦1.2 mm Hg, for example. If pumping capacity greatlyexceeds leak rate it should be possible to achieve an absolute pressurelow enough that no correction of optical path length is needed tocompensate for residual gases in the sheath.

The arrangement of FIG. 3 is impractical for measuring the position of astage which must be free to move in the X and Y directions, as mirror170 must be free to travel relative to interferometer 160 in the X and Ydirections without breaking vacuum or disturbing measurement laser beam300. FIG. 4 shows an idealized modification of the arrangement of FIG. 3in which a laser sheath 400 surrounding measurement beam 185 is made upof a fixed sheath portion 405, a movable sheath portion 410 whichtravels with stage bed 110 as wafer 100 is positioned along the Xdirection, a sliding seal 415 which allows movement of movable sheathportion in the X direction without loss of vacuum, and a sliding seal420 which allows mirror 170 to slide relative to sheath portion 410 asbed 120 is moved in the Y direction. A vacuum pump 425 maintains vacuumwithin the sheath.

The arrangement of FIG. 4 is also not ideal, as it is undesirable formovable sheath portion 410 or seal 420 (or anything else) to contact anypart of the precision stage. Among other things, such contact may applya force which can affect stage positioning and which can cause the stagepositioning motors to be continually under load. It is preferable tomeasure the position of the stage without any possibility of affectingstage position and without loading the stage drive motors.

FIG. 5 shows a modified arrangement for measuring stage position withoutapplying a force to the stage. A laser sheath 500 surroundingmeasurement beam 185 is made up of a fixed sheath portion 505, a movablesheath portion 510 which travels with stage bed 110 as wafer 100 ismoved along the X direction, a sliding seal 515 which allows movement ofmovable sheath portion 510 in the X direction without loss of vacuum,and a window 520 through which measurement beam 185 passes to reachmirror 170. Movable sheath portion 510 is positioned in the X directionby a stage follower motor 525. Stage follower motor 525 drives aturn-screw 530 for moving a threaded follower nut 535 affixed to movablesheath portion 510. Stage follower motor 525 is operated by a controlsignal received via line 540 from stage follower control electronics545. A proximity sensor 550 mounted adjacent window 520 supplies agap-distance signal via line 555 to stage follower control electronics545 so that the gap between stage mirror 170 and window 520 ismaintained constant as movable sheath portion 510 follows movement ofstage bed 110 in the X direction. A vacuum pump (not shown) maintainsvacuum within sheath 500.

Interferometer measurement electronics 560 of interferometer 160 supplya raw measured-distance signal (representing the optical path lengthbetween interferometer 160 and mirror 170) via line 565 tomeasured-distance correction electronics 570. Measured-distancecorrection electronics 570 also receive a pressure signal from pressuresensor 575, a gap-air temperature signal from temperature sensor 580,and the gap-distance signal from proximity sensor 550. Measured-distanceelectronics 570 correct the raw measured-distance signal for the effectsof window 520, of the air gap between window 520 and mirror 170, and ofthe pressure within sheath 500, and supply a correcteddistance-measurement signal to system stage control electronics 585 andto system control electronics 590. System control electronics 590 supplystage control signals to system stage control electronics 585 forcontrol of the X-axis stage motor.

FIG. 6A shows schematically the path of measurement laser beam 185 inthe arrangement of FIG. 5. Distance L_(V) represents the measurementpath length within the sheath from interferometer 160 to window 520,distance L_(W) represents the measurement path length through window520, and distance L_(A) represents the measurement path length throughthe air gap. The optical path length from interferometer 160 to mirror170 can be expressed as:

    OPL=L.sub.V η.sub.V =L.sub.W η.sub.W +L.sub.A η.sub.A

where η_(V) is the index of refraction of the vacuum environment withinthe sheath, η_(W) is the index of refraction of the window material andη_(A) is the index of refraction of the air gap.

FIG. 6B shows schematically the effect of variation in the air-gap pathlength by an amount ΔL, which can occur if the stage follower does notperfectly follow movement of mirror 170. The optical path length can beexpressed as:

    OPL=(L.sub.V -ΔL)η.sub.V +L.sub.W η.sub.W +(L.sub.A+ΔL)η.sub.A

or

    OPL=L.sub.A η.sub.A +L.sub.W η.sub.W +L.sub.V ηV+ΔL(η.sub.A -η.sub.V)

If distance ΔL and indices η_(A) and η_(AV) are known precisely, theeffect of variation in the air-gap path length can be compensated.Distance ΔL is determined by proximity sensor 550, which can be aninductive or capacitive sensor. For example, it is believed thatdistance ΔL can be determined to within about 3.4 μm using a suitableposition sensor such as a series SMU-9000 sensor from KamanInstrumentation Corporation of Colorado Springs, Colo. Index η_(A)varies mainly with changes in ambient temperature, and is compensatedusing the output signal from temperature sensor 580. Index η_(V) variesmainly with changes in pressure within sheath 500, and is compensatedusing the output signal from pressure sensor 575.

FIG. 7 shows a modified version of the arrangement of FIG. 5 asinstalled in a photolithography system. A wafer 100 is to be positionedrelative to an image from projection lens assembly 145. Aninterferometer 700 emits a measurement laser beam 185, and aninterferometer 710 emits a reference laser beam 705 which is directed ata mirror 715 mounted on the housing of lens assembly 145. A sheath 720comprises a movable sheath portion 510, a housing 725 and a fixed sheathportion 730. Movable sheath portion 510 encloses the path of measurementlaser beam 185 up to window 520. Fixed sheath portion 730 encloses thepath of reference laser beam 705 up to window 735. Fixed sheath portion730 is held in position by a bracket 740 such that an air gap 745 ismaintained between window 735 and mirror 715. Air gap 745 is set equalto the nominal air gap between window 520 and mirror 170, so thatreference laser beam 705 is subjected to virtually the same optical-pathinfluences as is measurement laser beam 185. The apparent distancebetween interferometer 700 and mirror 170 is unaffected by anyvariations which affect the optical path of both beams. Distance Lbetween mirror 715 and mirror 170 varies with movement of stage bed 110.The system is preferably designed such that distance is approximatelyzero when stage bed 110 is in the center of its range of travel. Thebeam from splitter 155 is supplied to interferometer via a mirror 750, awindow 755, and a splitter 760.

FIG. 8 shows the installation of a pair of interferometer systems likethat of FIG. 7 in a wafer exposure system having an XY stage. Inaddition to X-axis sheath 720, a Y-axis sheath 800 comprises a movablesheath portion 810, a housing 815 and a fixed sheath portion 820.Movable sheath portion 810 encloses most of the path of a Y-axismeasurement beam 190. Fixed sheath portion 820 encloses most of the pathof a Y-axis reference beam (not shown) comparable to reference beam 705.

Laser sheaths in accordance with the invention are useful not only withtraditional stage systems of the kind shown in FIG. 1. They are alsouseful, for example, with a guideless XY stage of the kind disclosed inU.S. patent application Ser. No. 08/221,375, filed Apr. 1, 1994,entitled "Electro-Mechanical Alignment and Isolation Method andApparatus," now U.S. Pat. No. 5,528,118 issued Jun. 18, 1996, thecontent of which is incorporated herein by this reference.

FIG. 9A is an exploded, perspective view of portions of a guidelessstage. A stage 900 carries a wafer 902. A follower assembly 904surrounds stage 900. As shown in the assembled, perspective view of FIG.9B, stage 900 is supported above an air-bearing plate 906 on a cushionof air provided by an air/vacuum bearing. Air beating plate 906 issupported by a base 908. Stage 900 carries magnetic drive coils oflinear drive motors used for moving stage 900 in an X-Y plane; a coil910 forms part of one of two linear drive motors for moving stage 900 inthe X direction and coils 912 and 914 form part of linear drive motorsfor moving stage 900 in the Y direction. Stage 900 also carriers mirrors916 and 918 for determining position in the X and Y directions androtational orientation Θ about the Z-axis with laser interferometers.

Stage 900 is followed closely by an X follower and a Y follower. The Yfollower includes a cross member 942 connected to a pair of spaced apartarms 944 and 946. Arm 944 has a drive track 948 which cooperates with acoil (not shown) of stage 900 to form a first linear motor. Arm 946 hasa drive track (not shown) which cooperates with coil 910 to form asecond linear motor. The X follower includes a cross member 950connected to a pair of spaced-apart arms 952 and 954. Arm 952 has adrive track (not shown) which cooperates with coil 912 to form a thirdlinear motor. Arm 954 has a drive track 956 which cooperates with coil914 to form a fourth linear motor.

The X follower and Y follower are supported on a reaction frame. Thereaction frame can be supported independently of base 908. Ends 958 and960 of arms 946 and 944 ride on a rail 962 of the reaction frame, whileends 963 and 964 of arms 944 and 946 ride on an opposing rail (notshown) of the reaction frame. A drive mechanism (not shown) moves the Yfollower in the Y direction. Ends 965 and 966 of arms 952 and 954 rideon rails 967 and 968 of the reaction frame, while ends 970 and 972 rideon opposing rails (not shown) of the reaction frame. A drive mechanism(not shown) moves the X follower in the X direction. The drivemechanisms used to move the X and Y followers are used for coarsepositioning and can thus be non-precision devices such as drive screwsor drive motors using roller guides. While the linear drive motors movestage 900, the X and Y followers maintain themselves at a small distancefrom stage 900 without contacting stage 900. The linear-motor coils ofstage 900 ride inside the drive tracks of the followers, but do notphysically contact the followers or anything else. See, for example, thedetail view of FIG. 14 showing coil 920 within (but not touching)follower arm 946.

FIGS. 10A-10D are a sequence of perspective views showing manipulationof the X and Y followers as stage 900 moves around. From FIG. 10A toFIG. 10B, X follower 1010 moves in the X direction to closely followstage 900 as it travels in the X direction. From FIG. 10B to FIG. 10C, Yfollower 1020 moves in the Y direction to closely follow stage 900 as ittravels in the Y direction. From FIG. 10A to FIG. 10D, stage 900 movesfrom one extreme position to the opposing extreme position in the X andY directions. Whatever the position of the stage, the X and Y followersfollow in close proximity.

FIGS. 11-12 show such a guideless stage to which interferometers withsheaths have been added in accordance with the invention. Movement ofstage 900 is to be controlled for positioning a wafer 902 relative to anoptical image produced by a light source 1100, a reticle 1102 and aprojection lens assembly 1104 in a wafer exposure system. Source 1106produces a laser beam 1108 which is split by a splitter 1110 into beams1112 and 1114.

Beam 1112 is supplied via a mirror 1116 to a Y-axis interferometersystem 1118 having a Y-axis measurement laser beam directed at mirror918 of stage 900 and a reference laser beam directed at a mirror on lensassembly 1104. A laser sheath assembly is provided having a housing1120, a movable sheath portion 1122 and a fixed sheath portion 1124. Asshown in FIG. 12, movable sheath portion 1122 encloses the path of theY-axis measurement laser beam up to a window 1126 which is spaced frommirror 918 by an air gap 1128. Movable sheath portion 1122 is affixed toarm 946 of the Y follower by a mounting block 1130 and enters housing1120 through a sliding seal so that movable sheath portion 1122 extendsand retracts as the Y follower is moved. Movable sheath portion 1122 iscarded by Y-follower arm 946 and thus does not require a separate stagefollower motor. Variations in air gap 1128 can occur due to followererror (variation in the distance at which the Y follower follows stage900. These variations are small and are readily compensated as describedabove. Fixed sheath portion 1124 encloses the path of the Y-axisreference laser beam up to a window 1132 which is spaced from a mirror1134 on lens assembly 1104 by an air gap 1136. Housing 1120, sheathportion 1124 and lens assembly 1104 are fixed. Air gap 1136 thus remainsconstant and is set equal to the nominal air gap 1128 so the Y-axisreference laser beam is subjected to virtually the same optical-pathinfluences as is the Y-axis measurement laser beam.

Beam 1114 is supplied via a minor 1170 to an X-axis interferometersystem 1172 having dual interferometers spaced apart along the X axisfor determining position and yaw (rotation) of stage 900. A first X-axisreference laser beam is directed at mirror 916 and a first X-axismeasurement laser beam is directed at a mirror 1174 on lens assembly1104. A second X-axis reference laser beam is directed at mirror 916 anda second X-axis measurement laser beam is directed at a mirror (notshown) on lens assembly 1104. An X-axis laser sheath includes a housing1176, a movable sheath portion 1178, a fixed sheath portion 1180, amovable sheath portion 1182 and a fixed sheath portion 1184.

Movable sheath portion 1178 encloses the path of the first X-axismeasurement laser beam up to a window 1186 which is spaced from mirror916 by an air gap (not shown). Movable sheath portion 1178 is affixed toarm 954 of the X follower by a mounting block 1188 and enters housing1176 through a sliding seal 1190 so that movable portion 1178 extendsand retracts as the X follower is moved. Movable sheath portion 1182encloses the path of the second X-axis measurement laser beam up to awindow 1192 which is spaced from mirror 916 by an air gap (not shown).Movable sheath portion 1182 is affixed to arm 954 of the X follower by amounting block 1194 and enters housing 1176 through a sliding seal 1196so that movable sheath portion 1182 extends and retracts as the Xfollower is moved. Movable sheath portions 1178 and 1182 are carried byX-follower arm 954 and thus do not require a separate stage followermotor. Variations in air gaps between mirror 916 and windows 1186 and1192 are small, and result only from X follower error (variation in thedistance at which the X follower follows stage 900). Fixed sheathportion 1180 encloses the path of the first X-axis reference laser beamup to a window 1198 which is spaced from mirror 1174 by an air gap (notshown). Housing 1176, sheath portion 1180 and assembly 1104 are fixed.The air gap between window 1198 and mirror 1174 thus remains constantand is set equal to the nominal air gap between window 1186 and mirror916 so the X-axis reference laser beam is subjected to virtually thesame optical-path influences as is the X-axis measurement laser beam. Asecond Y-axis interferometer can be readily added to the system of FIGS.11-12, and is useful for calibration and redundancy.

For simplicity, FIGS. 5 and 7 and the above discussion suggestinterferometers having a measurement beam which makes a single roundtrip to the stage mirror. It is preferred, however, to use laserinterferometers in which the measurement beam makes two round trips fromthe interferometer. This provides increased measurement resolution, as agiven displacement of the stage mirror produces twice as manyinterference fringe counts. These are also much less sensitive tomisalignment errors. The beams are illustrated as elliptical but couldas well be circular. An example of an interferometer system of this typeis the HP 5527B laser interferometer positioning system availablecommercially from Hewlett-Packard Company of Mountain View, Calif., USA.It is possible to use such interferometers in the arrangement of FIGS.11-12 if the internal cross-sectional dimension of the sheaths is largeenough to accommodate multiple beams, e.g., if the internal diameter ofeach of sheath portions 1122, 1178 and 1182 and of each of sheathportions 1124, 1180 and 1184 is large enough for two beams.

FIGS. 13-16 show a system similar to that of FIGS. 11-12, modified tohave a separate sheath portion for each of four measurement beams andfor each of four reference beams in a single axis (e.g., two measurementbeams and two reference beams for each of two Y-axis interferometers, toprovide high-resolution yaw and position measurement). FIG. 13 is apartially cut-away elevation view showing additional structuralelements, in which base 908 is suspended by a plurality of support arms,such as support arm 1350, from an inertial bridge 1352. Inertial bridge1352 is in turn supported on isolation pads (not shown) and carriesprojection lens assembly 1104. Air bearing plate 906 is supported onbase 908 by a plurality of mounts, such as mount 1354 and mount 1356,which are adjustable in the z-direction to set the elevation and tilt ofair bearing plate 906 and thus of stage 900. Stage 900 has openings (notshown) in its lower surface through which pressurized air is released tosupport stage 900 on a cushion of air above air bearing plate 906. FIG.13 also shows principal laser-sheath elements. A vacuum pump 1300 isconnected by tubing 1302 to a housing 1120'. A pressure sensor 1304monitors pressure in housing 1120', and a leak-back pressure controller1306 maintains a constant pressure in housing 1120'. An interferometer1308 is one of two Y-axis interferometers. One measurement beam 1310 andone reference beam 1312 of interferometer 1308 are shown in FIGS. 13-14.Measurement beam 1310 passes through a movable sheath portion 1122'which is affixed by a support block 1360 to arm 946 of the Y follower.The Y follower is in the far right position in FIGS. 13-14 so thatmovable sheath portion 1122' is fully retracted. Reference beam 1312passes within fixed sheath portion 1124' and through window assembly1132' and air gap 1136' to mirror 1134' on lens assembly 1104.

As seen more clearly in the enlarged-scale sectional view of FIG. 14,fixed sheath portion 1124' is mounted in an opening 1400 of housing1120' with a seal 1405. Movable sheath portion 1122' is mounted in abearing assembly 1410 which passes through the wall of housing 1120' andwhich allows free axial movement of sheath portion 1122'. Window 1126'is affixed to and travels with block 1360 as arm 946 moves. The windowend 1415 of movable sheath portion 1122' is coupled by a flexible joint1420 (e.g., of molded urethane) to a nipple 1425 fitted to block 1360,allowing movable sheath portion 1122' to align itself within beating1410. Also shown in FIG. 14 are proximity sensors 1430 and 1435 formeasuring air gap 1128'.

FIG. 15 is a sectional view taken along line XV--XV of FIG. 14, showinga vertical face of block 1360 with four measurement beams 1500, 1502,1310 and 1504, with four reference-beams 1506, 1508, 1312 and 1510housed in respective fixed-sheath portions 1512, 1514, 1124' and 1516,with three proximity sensors 1430, 1435 and 1530, and with two air-gaptemperature sensors 1535 and 1540. Multiple proximity sensors andtemperature sensors allow precise determination of the air-gap distanceand temperature for each measurement beam.

FIG. 16 is a sectional view taken along line XVI--XVI of FIG. 14,showing a vertical face of housing 1120'. Beating 1410 supports movablesheath portion 1122' in the wall of housing 1120'. Similarly, beatings1600, 1605 and 1610 support respective movable sheath portions ofmeasurement beams 1500, 1502 and 1504. Beatings 1410, 1600, 1605 and1610 are preferably air beatings, each having a pressure input and avent, such as pressure input 1620 and vent 1615 of beating 1410.

FIG. 17 shows beating 1410 in an enlarged cross-sectional elevation viewtaken through line XVII--XVII of FIG. 18. FIG. 18 shows beating 1410 inan enlarged end view taken as in FIG. 16, but with movable sheathportion 1122' removed. Bearings 1600, 1605 and 1610 are of identicalconstruction. Bearing 1410 is mounted in an opening in the wall ofhousing 1120' with a flexible sealing material 1700 (e.g., of Devcon)which allows bearing 1410 to align itself with the axis of movablesheath portion 1122'. Air is supplied under pressure to input opening1620 and passes via a manifold 1780 and bores 1755, 1760, 1765 and 1770into the annular space between beating 1410 and movable sheath portion1122'. Air returns to vent 1615 via annular grooves 1725, 1730 and 1735,longitudinal grooves 1740, 1745 and 1750, and bores 1710, 1715 and 1720.With movable sheath portion 1122' in place, air flows between the outersurface of movable sheath portion 1122' and the inner surface of bearing1410 to provide support with free axial movement within beating 1410.The flow of air is indicated by arrows in FIGS. 17 and 18. Most of theair flow is recovered via vent 1615; the small amount which leaks tohousing 1120' is negligible with respect to the capacity of the vacuumpump.

Other arrangements within the spirit and scope of the invention can beused to maintain the path of the measurement laser beam largely within acontrolled environment. FIG. 19 shows an example in which measurementbeam 1900 of an interferometer 1905 is enclosed in a sheath comprising ahousing 1910, a tube 1915 which carries a laser window 1920 and aproximity sensor 1925 and temperature sensor 1930, and a supportedbellows assembly 1935 connecting tube 1915 to housing 1910. Bellowsassembly 1935 includes a bellows 1940 and, preferably, a support tube1945 which telescopes into movable tube 1915. A motor 1950 drives screw1955 to move block 1960 for extending and retracting the sheath. Forsimplicity of illustration, interferometer 1905 is of the type having aninternal reference beam. The sheath arrangement of FIG. 19 can also besubstituted for the sliding tube arrangements of the embodimentsdescribed above.

In the embodiments described above, a window is provided at the stageend of the sheath to maintain the environment in the sheath whileavoiding contact with the stage. FIG. 20 shows a modification inaccordance with the invention in which movable sheath portion 1122' isfitted at the stage end with a vacuum/air bearing tip 2000 which floatson a cushion of air adjacent mirror 918. A beating end 2005 has acentral bore 2010 through which measurement beam 1310 passes, apressurized-air inlet 2015 which feeds an annular channel 2025 and avent outlet 2020 which draws air from annular channel 2030. Air flowfrom channel 2025 creates an air bearing which keeps face 2035 fromcontacting mirror 918. Bearing end 2005 is supported by a flexiblediaphragm 2040 on a nipple 2045 affixed to tube end 2050. Diaphragm 2040maintains a vacuum seal between bearing end 2005 and nipple 2045, whileallowing bearing end 2005 to be resiliently biased toward mirror 918 bycoil spring 2055. Proximity sensor 2060 supplies a signal to controlelectronics (e.g., as in FIG. 5) so that a stage follower can maintainsensor 2060 at a constant distance from minor 918. The force of spring2055 and air pressure to inlet 2015 and venting to outlet 2020 arebalanced to allow bearing end 2005 to float as an air bearing a smalldistance from mirror 918. This arrangement has the advantage thatcompensation for the air gap and window of the other embodiments is notrequired. While some air flowing from channel 2025 may leak into movablesheath portion 1122', the amount will be negligible with respect to thevacuum pump capacity and will not materially affect the optical pathlength of measurement beam 1310.

It may be desirable in some installations to reduce the overall"footprint" of the system without limiting the travel distance of thestage. FIG. 21 shows the horizontal extent of a single-axis sheath 500relative to a stage mirror 170 as in the embodiment of FIG. 5. Thehorizontal extent of the system can be reduced by substituting forsingle-piece sheath portion 510 a telescoping sheath assembly havingmultiple tubes which can be retracted as shown in FIG. 22 and extendedas shown in FIG. 23. A first tube 2200 is supported for free axialmovement in an air-guided bearing 2205 mounted in wall 2210 of housing2215. A second tube 2220 is supported for free axial movement in anair-guided bearing 2225 at the stage end of first tube 2200. A firstfollower motor 2245, screw 2250 and support 2255 form a drive system forfirst tube 2200. A second follower motor 2230, screw 2235 and support2240 form a drive system for second tube 2220. Motors 2230 and 2245 arecontrolled in response to the output of a proximity sensor 2260 tomaintain the stage end of the telescoping sheath assembly at a fixeddistance from stage mirror 170.

Other modifications are also possible. It is an aim of the invention toenclose substantially all the measuring portions of a laserinterferometer's measurement path in a common environment, whileallowing measurements to be made on the position of a stage which isoutside this protected environment. While the embodiments describedabove enclose the path in a vacuum, other environments can besubstituted, such as a well-controlled, gas atmosphere. For example,FIG. 24 shows a modified version of the arrangement of FIG. 13 in whichmost of the measurement path is enclosed in a helium-gas-filled sheath.

A helium environment has a number of advantages relative to an airenvironment. Helium has one-eighth the index of refraction relative to avacuum as does air, reducing environmentally-related error-causingdisturbances such as variations in temperature, pressure and humidity.In addition, helium has higher thermal conductivity, so thermalgradients and temperature fluctuations are expected to be much smaller.A gas environment also has some advantages relative to a vacuumenvironment. Mechanical forces caused by pressure differential betweenthe sheath environment and the atmospheric environment are reduced, thusreducing the design requirements of elements such as window 1132'. A gasenvironment can also help with transmission of heat generated by theoptical system.

Changing the described vacuum-sheath systems to gas-sheath systemsraises several issues. First, it is desired to maintain gas density(temperature, pressure, humidity and homogeneity of gas composition)reasonably uniform over the measurement path and, preferably, throughoutthe sheath. This can be done by providing a system for recirculating thegas within the sheath and compensators such as a local-pressure bufferand a weighted volume monitor for maintaining gas pressure within thesheath. Second, it is preferable to maintain the gas pressure aboveatmosphere pressure to minimize entry of air which would contaminate thegas, but only slightly above atmospheric pressure to minimize gasleakage from the sheath. The sheath pressure can be, for example, justone psi or so greater than atmospheric. Sheath overpressure and leakageare particularly important with sliding seals (such as the air-bearingseal of FIG. 17). A gas-supply source can accommodate leakage. Third, itis preferable to monitor properties of the gas environment using, forexample, a wavelength tracker which provides a compensation signal tothe measurement-correction electronics.

Referring to FIG. 24, a helium chamber 2400 encloses a low-pressurecirculation fan 2402, a wavelength tracker 2404, and a pressure/volumecontroller 2406. Wavelength tracker 2404 can be any suitable unit, suchas a model HP10717A tracker available commercially from Hewlett-PackardCo., supplying a further correction signal to measured-distancecorrection electronics 570. Pressure/volume controller 2406 is of anysuitable design; as schematically illustrated in FIG. 24, it comprises aweighted diaphragm assembly 2408, a volume-level detector 2410, and avent 2412 to atmosphere. Detector 2410 controls a controllable valve2414 to allow gas to enter chamber 2400 from a helium-supply container2416 as needed to replenish the chamber.

Flexible umbilical tubes convey gas from fan 2402 to the housings of thestage's interferometer sheaths and from these housings back to chamber2400. As shown, flexible tube 2418 conveys helium from fan 2402 tohousing 1120', and a flexible tube 2420 conveys helium to the housingsof other sheaths (e.g., X-axis sheaths not shown in FIG. 24). A tube2422, through which tube 2418 passes, is of sufficiently large diameterto provide a return path for helium from housing 1120' to chamber 2400.Similarly, a tube 2424, through which tube 2420 passes, provides areturn path for helium from the housings of the other sheaths. Tube 2418branches within chamber 1120' to supply helium via a recirculation tube2426 to the distal end of fixed sheath portion 1124', so that heliumwithin fixed sheath portion 1124' is continuously refreshed along thereference beam path. It is also preferable to branch tube 2418 withinchamber 1120' to supply helium via a recirculation tube or channel (notshown) passing within movable sheath portion 1122' to the distal end ofmovable sheath portion 1122' so that helium flow is also maintainedalong the measurement beam path. Movable sheath portion 1122' and therecirculation tube or channel (not shown) which delivers gas to thedistal end of movable sheath portion 1122' are designed so as not tointerfere with the measurement beam. The volume of the sheath changes asmovable sheath portion 1122' extends and retracts with movement of stage900. To minimize pressure variations which could affect the measuredoptical path length, these volume changes are compensated by a localvolume buffer 2428 having a spring-loaded diaphragm 2430 and a vent 2432to atmosphere so that the upper surface of diaphragm 2430 is exposed toatmospheric pressure, the same pressure to which vent 2412 is exposed.

Maintaining one environment (temperature, pressure, humidity) for alllaser interferometer measurement paths in the system improves accuracyand stability of distance measurements. Having the same controlledenvironment for all interferometers of a system also enables smallerrelative measurement errors between interferometers. With such a system,the design of air handling in the vicinity of the laser interferometersis not constrained by laser measurement issues because the measurementpaths are enclosed. Outside sources of turbulence, temperaturevariation, humidity, etc., thus do not interfere with the measuringfunction of the interferometer.

The embodiments described apply the inventive concepts to two differenttypes of stage structure: a conventional stage to which followers areadded for positioning the movable sheath portions (e.g., as in FIG. 8),and a guideless stage already having followers to which the movablesheath portions cad be attached (e.g., as in FIG. 11). Laser sheaths inaccordance with the invention can be applied to other stage structuresas well. Consider, for example, the stage structure disclosed in U.S.patent application Ser. No. 08/220,740 filed Oct. 19, 1994 of T. Novaket al. entitled "Precision Motion Stage now U.S. Pat. No. 5,623,853issued Apr. 29, 1997, with Single Guide Beam and Follower Stage," thecontent of which is incorporated herein by this reference. The latterstage structure has a stage follower in the X direction and has nofollower in the Y direction. In such a "hybrid" stage structure, a lasersheath for the X axis interferometer can have a movable sheath portionattached to the existing stage follower (e.g., as in FIG. 11). Afollower can be added (e.g., as in FIG. 8) for keeping a movable sheathportion of the laser sheath for the Y axis interferometer at a constantdistance from the stage.

The foregoing description of preferred embodiments of the invention isintended as illustrative only, and not as a limitation of the inventionas defined by the claims which follow. Those of skill in the art willrecognize many modifications which may be made in the preferredembodiments within the spirit and scope of the claimed invention.

We claim:
 1. Apparatus for measuring position of a stage, comprising:a.a stage movable along mutually-orthogonal first and second axes, thestage having a first-axis mirror and a second-axis mirror; b. a firstlaser interferometer for directing a first-axis measurement beam along afirst-axis beam path toward the first-axis mirror and for producing afirst measured-distance signal indicative of an optical path lengthbetween the first laser interferometer and the first-axis mirror; c. asheath enclosing the first-axis measurement beam along substantially theentire first-axis beam path wherein the sheath defines an interiorvolume such that an atmosphere outside the interior volume is preventedfrom entering the sheath, the sheath including a first-axis sheathportion having an effective length which varies in response to afollower system as the first-axis mirror changes position; d. a secondlaser interferometer for directing a second-axis measurement beam alonga second-axis beam path toward the second-axis mirror and for producinga second measured-distance signal indicative of an optical path lengthbetween the second laser interferometer and the second-axis mirror; e. asheath enclosing the second-axis measurement beam along substantiallythe entire second-axis beam path wherein the sheath defines an interiorvolume such that an atmosphere outside the interior volume is preventedfrom entering the sheath, the sheath including a second-axis sheathportion having an effective length which varies in response to afollower system as the second-axis mirror changes position; f. afirst-axis follower system for varying the effective length of thefirst-axis sheath portion so that substantially the entire first-axisbeam path remains enclosed as the first-axis mirror changes positionrelative to the first laser interferometer, wherein the first-axisfollower system comprises a follower which follows movement of the stagewhile remaining out of contact with the stage, and wherein an end of thefirst-axis sheath portion nearest the first-axis mirror is mounted formovement with the follower such that said end of the first-axis sheathremains at a substantially constant spacing from the first-axis mirror;and g. a second-axis follower system for varying the effective length ofthe second-axis sheath portion so that substantially the entiresecond-axis beam path remains enclosed as the second-axis mirror changesposition relative to the second laser interferometer.
 2. The apparatusof claim 1, further comprising:h. an environmental control system forcontrolling an environment within the sheaths.
 3. The apparatus of claim2, further comprising a correction system for detecting characteristicsof the second-axis beam path and for compensating the secondmeasured-distance signal in response to the detected characteristics toproduce a second corrected signal indicative of actual distance betweenthe second laser interferometer and the second-axis mirror.
 4. Theapparatus of claim 1, further comprising a correction system fordetecting characteristics of the first-axis beam path and forcompensating the first measured-distance signal in response to thedetected characteristics to produce a first corrected signal indicativeof actual distance between the first laser interferometer and thefirst-axis mirror.
 5. The apparatus of claim 1, wherein the second-axisfollower system comprises a follower which follows movement of the stagewhile remaining out of contact with the stage, and wherein an end of thesecond-axis sheath portion nearest the second-axis mirror is mounted formovement with the follower such that said end of the second-axis sheathremains at a substantially constant spacing from the second-axis mirror.6. The apparatus of claim 1, further comprising a window covering an endof the first-axis sheath portion nearing the first-axis mirror andsealing the interior volume of the first-axis sheath.
 7. The apparatusof claim 1, further comprising a bearing at an end of the first-axissheath portion nearest the first-axis mirror and sealing the interiorvolume of the first-axis sheath by bearing against the first-axismirror.
 8. Apparatus for measuring position of a stage movable alongmutually-orthogonal first and second axes, the stage having a firstreflector substantially normal to the first-axis, comprising:a. afirst-axis interferometer for directing a first-axis measurement beamalong a first beam path parallel to the first-axis toward the firstreflector and for producing a measured-distance signal indicative of anoptical path length between the first-axis interferometer and the firstreflector; b. a first-axis follower system having a first-axis followerwhich follows movement of the stage along the first-axis while remainingout of contact with the stage; and c. a first-axis sheath enclosing thefirst-axis measurement beam along a substantial part of the first beampath wherein the sheath defines an interior volume such that anatmosphere outside the sheath is prevented from entering the sheath, thefirst-axis sheath including a movable sheath portion carried by thefollower such that the sheath has an effective length which varies withmovement of the stage along the first-axis while remaining out ofcontact with the stage.
 9. The apparatus of claim 8, wherein thefirst-axis follower maintains an end of the first-axis sheath at asubstantially constant spacing from the first reflector.
 10. Theapparatus of claim 9, wherein the stage has a second reflectorsubstantially normal to the second-axis, further comprising:d. asecond-axis interferometer for directing a second-axis measurement beamalong a second beam path parallel to the second-axis toward the secondreflector and for producing a measured-distance signal indicative of anoptical path length between the second-axis interferometer and thesecond reflector; e. a second-axis follower system having a second-axisfollower which follows movement of the stage along the second-axis whileremaining out of contact with the stage; and f. a second-axis sheathenclosing the second-axis measurement beam along a substantial part ofthe second beam path wherein the sheath defines an interior volume suchthat an atmosphere outside the sheath is prevented from entering thesheath, the second-axis sheath including a movable sheath portioncarried by the follower such that the second-axis sheath has aneffective length which varies with movement of the stage along thesecond-axis while remaining out of contact with the stage.
 11. Theapparatus of claim 10, wherein the second-axis follower maintains an endof the second-axis sheath at a substantially constant spacing from thesecond reflector.
 12. The apparatus of claim 9, further comprising anenvironmental controller for controlling an environment within thefirst-axis sheath.
 13. The apparatus of claim 12, wherein theenvironmental controller comprises a vacuum pump for evacuating thefirst-axis sheath and pressure controller for maintaining pressurewithin the first-axis sheath within predetermined limits.
 14. Theapparatus of claim 9, further comprising a corrector for detectingcharacteristics of the first beam path and for compensating themeasured-distance signal in response to the detected characteristics toproduce a corrected signal indicative of actual distance between thefirst-axis interferometer and the first reflector.
 15. The apparatus ofclaim 14, further comprising means for maintaining a substantiallyconstant gas pressure within the first-axis sheath wherein the gasdiffers from an atmosphere outside the first-axis sheath.
 16. Theapparatus of claim 9, wherein the first-axis sheath comprises a housingenclosing a portion of the first beam path, the housing having a walland a beam opening through the wall, and wherein the sheath portioncomprises an elongate hollow member extending through the beam openingand supported for movement relative to the wall in a direction generallyparallel to the first beam path.
 17. The apparatus of claim 16, furthercomprising an air-guided bearing for supporting the sheath portion inthe beam opening.
 18. The apparatus of claim 16, wherein an end of theelongate hollow member nearest the first reflector is fitted with abearing for maintaining the sheath portion at a substantially fixedspacing from the reflector.
 19. The apparatus of claim 18, wherein thebearing comprises a tip end having a face resiliently biased toward thereflector and having at least one channel for expelling pressurized gasto maintain separation between the face and the first reflector.
 20. Theapparatus of claim 8, wherein the first-axis sheath portion comprises ahousing enclosing a portion of the first beam path, the housing having awall and a beam opening through the wall, wherein the first-axis sheathportion comprises an elongate hollow member extending through the beamopening and supported for movement relative to the wall in a directiongenerally parallel to the first beam path, and wherein the first-axissheath portion further comprises a window covering an end of theelongate hollow member nearest the first reflector, the window allowingpassage of the first-axis measurement beam between the interior volumeof the sheath and the first reflector.
 21. The apparatus of claim 20,wherein the follower varies the effective length of the sheath portionby displacing the elongate hollow member along the first beam path tomaintain the window at a substantially constant distance from the firstreflector.
 22. The apparatus of claim 20, further comprising a gapsensor for measuring distance between the window and the first reflectorand wherein the corrector is responsive to the gap sensor forcompensating the measured-distance signal in dependence on measureddistance between the window and the reflector.
 23. The apparatus ofclaim 22, further comprising a temperature sensor for detectingatmospheric temperature in the region of a gap between the window andthe reflective surface and wherein the corrector is responsive to thetemperature sensor for compensating the measured-distance signal independence on the detected atmospheric temperature.
 24. The apparatus ofclaim 20, further comprising a pressure sensor for detecting pressurewithin the interior volume of the sheath and wherein the corrector isresponsive to the pressure sensor for compensating the measured-distancesignal in dependence on the detected pressure.
 25. The apparatus ofclaim 20, wherein the window has an index of refraction which differsfrom the index of refraction of the interior volume of the sheath, andwherein the corrector compensates the measured-distance signal for theoptical path length of the window.
 26. The apparatus of claim 8, whereinthe first-axis sheath comprises a housing enclosing a portion of thefirst beam path, the housing having a wall and a beam opening throughthe wall, and wherein the sheath portion comprises a telescopingassembly of elongate hollow members, the assembly extending through thebeam opening and mounted for movement relative to the wall in adirection generally parallel to the first beam path.
 27. The apparatusof claim 8, wherein the sheath comprises a housing enclosing a portionof the beam path, the housing having a wall and a beam opening throughthe wall, and wherein the sheath portion comprises an elongate hollowmember having a bellows of variable length, the sheath portion mountedin the opening for extension and retraction relative to the wall in adirection generally parallel to the beam path.
 28. The apparatus ofclaim 8, wherein the first-axis sheath is filled with a gas differingfrom an atmosphere outside the first-axis sheath.
 29. The apparatus ofclaim 28, further comprising means for recirculating gas within thefirst-axis sheath to maintain a substantially uniform gas density withinthe first-axis sheath.
 30. The apparatus of claim 8, wherein thefirst-axis sheath is filled with helium.
 31. The apparatus of claim 8,wherein the first-axis sheath is filled with a gas, and furthercomprising a wavelength tracker for monitoring optical properties of thegas and for supplying a correction signal to said corrector.
 32. Theapparatus of claim 8, further comprising a window,covering an end of thefirst-axis sheath nearest the first reflector and sealing the interiorvolume of the first-axis sheath.
 33. The apparatus of claim 8, furthercomprising a bearing at an end of the first-axis sheath and sealing theinterior volume of the first-axis sheath.
 34. An exposure apparatus forexposing an image of a reticle onto a substrate mounted on an XY-stagewhich is movable in a predetermined X-Y plane, the apparatuscomprising:(a) a base assembly for supporting the XY-stage thereon tomove in the X-Y plane; (b) a frame structure having a rectangular frameassembly located around the base assembly and an X-follower assemblymovable in the X direction on the frame assembly so as to maintain apredetermined space from the XY-stage in the X direction when theXY-stage moves in the X direction; (c) a Y-drive actuator partiallymounted to the X-follower assembly for generating a force to move theXY-stage in the Y direction relative to the X-follower assembly on thebase assembly; (d) an X-axis interferometer system fixedly located withrespect to the base assembly for directing an X-axis measurement beamalong an X-axis beam path toward an X-axis mirror mounted on saidXY-stage and for producing an X-axis signal indicative of the positionof the XY-stage; and (e) an X-axis sheath enclosing the X-axismeasurement beam along a substantial part of the X-axis beam path,wherein the sheath defines an interior volume such that an atmosphereoutside the interior volume is prevented from entering the sheath, theX-axis including a movable sheath portion carried by the X-followerassembly such that the X-axis sheath has an effective length whichvaries with movement of the XY-stage along the X axis while remainingout of contact with the XY-stage.
 35. The exposure apparatus of claim34, wherein the frame structure further includes a Y-follower assemblymovable in the Y direction on the frame assembly so as to maintain apredetermined space from the XY-stage in the Y direction when theXY-stage moves in the Y direction.
 36. The exposure apparatus of claim35, further comprising:(f) an X-drive actuator partially mounted to theY-follower assembly for generating a force to move the XY-stage in the Xdirection relative to the Y-follower assembly on the base assembly; (g)a Y-axis interferometer system fixedly located with respect to the baseassembly for directing a Y-axis measurement beam along a Y-axis beampath toward a Y-axis mirror mounted on the XY-stage and for producing aY-axis signal indicative of the position of the XY-stage; and (h) aY-axis sheath enclosing the Y-axis measurement beam along a substantialpart of the Y-axis beam path, wherein the sheath defines an interiorvolume such that an atmosphere outside the interior volume is preventedfrom entering the sheath, the Y-axis sheath including a movable sheathportion carried by the Y-follower assembly such that the Y-axis sheathhas an effective length which varies with movement of the XY-stage alongthe Y-axis while remaining out of contact with the XY-stage.